1. Field of the Invention
The present invention relates to a particle beam therapy system which is provided with a synchrotron and is capable of precision irradiation for treatment.
2. Description of the Related Art
In the recent aging society, the radiotherapy for cancer treatment is attracting attentions because of its non-invasive nature and its good therapeutic effect for patients to maintain their high quality of life after treatment. Among various radiotherapy systems, the particle beam therapy system, which employs ion beams or beams of charged particles (such as protons or carbon atoms) accelerated by an accelerator, is considered particularly promising because of its ability to highly concentrate beams to the affected part. The particle beam therapy system is composed of an accelerator, a beam transport system, and an irradiation device. The first is a synchrotron or the like which accelerates beams of charged particles emerging from an ion source up to nearly the velocity of light. The second transports the beams of charged particles which have been extracted from the accelerator. The third irradiates a patient with beams of charged particles in a specific way according to the position and shape of the affected part. (See Patent Document 1.)
The conventional irradiation device attached to the particle beam therapy system expands the beam diameter by means of a scatterer and then scrapes off the periphery by means of a collimater to shape the beams of charged particles before irradiation with the beams of charged particles conforming to the shape of the affected part. Unfortunately, the irradiation method that employs a scatterer needs to improve the beam availability, to reduce the generation of neutrons, and to improve conformity between the irradiation region and the shape of the affected part. This has recently aroused a need for a method of more accurate irradiation, which is called scanning irradiation method. This method is made up of extracting thin beams of charged particles from an accelerator, deflecting them by means of electromagnets, and scanning the affected part with them in conformity with its shape.
The scanning irradiation method works to irradiate irradiation spots SP defined by division of the three-dimensional affected part into layers and further division of each layer into small segments. Layers in the depthwise direction are selectively irradiated with the beams of charged particles whose energy level is properly adjusted. Spots in each layer are irradiated with the beams of charged particles at a desired dose by two-dimensional scanning with electromagnets. The irradiation beams may be kept on or turned off during movement from one irradiation spot SP to another. These procedures are called raster scanning and spot scanning, respectively. The latter is disclosed in Patent Document 2.
The conventional spot scanning method will be described below with reference to FIG. 8. FIG. 8 shows the extraction preparing period that follows the completion of acceleration and the timing chart immediately after the start of extraction, which are involved in the operating sequence of the particle beam therapy system that employs the spot scanning method. The spot scanning method is made up of irradiating the irradiation spots sequentially one after another. While one spot is being irradiated at a prescribed dose, the beam scanning is suspended. The beam scanning is resumed, with the beams of charged particles turned off, and the adjacent spot is irradiated. Movement from one spot to another is accomplished by adjusting the amount of excitation for the scanning electromagnet. Irradiation in this manner determines the total dose to the affected part (or the irradiation field) in terms of the superposition of individual doses to individual irradiation spots SP.
The beams of charged particles are extracted from the synchrotron by the technique disclosed in Patent Document 3. This technique is made up of applying high-frequency waves to circling beams (those beams of charged particles which circle in the synchrotron), thereby increasing their amplitude of betatron oscillation, and extracting from the synchrotron those beam particles with the large amplitude of betatron oscillation exceeding the stability limit. This technique keeps constant during extraction the parameters for operation of the extraction-related apparatus of the synchrotron, resulting in a high orbit stability for the beams of charged particles being extracted from the synchrotron and a high position accuracy for the irradiation beams.    Patent Document 1: Japanese Patent No. 2833602    Patent Document 2: Japanese Patent No. 3874766    Patent Document 3: Japanese Patent No. 2596292
It is essential for the spot scanning technique to achieve precision irradiation for treatment to accurately position the beams of charged particles and to accurately control the beam size. With this in mind, the present inventors carried out experiments to realize a method of precision irradiation for treatment by the particle beam therapy system with a synchrotron. The results of experiments revealed that the beams of charged particles which have just been extracted from the synchrotron are poor in the reproducibility of the beam size and also poor in the stability of the beam orbit. The foregoing results may be discussed as follows.
In actual operation of a synchrotron, the beams of charged particles which have been introduced from a pre-accelerator become unstable in the course of acceleration to a prescribed energy level. Such beam instability easily occurs when the charged particles are at a low energy level immediately after their injection into the synchrotron, on account of repulsions (space charge effect) between particles constituting the beams of charged particles. As the result of beam instability, part of beam particles steeply increase in the amplitude of oscillation (betatron oscillation) with respect to the design orbit while they are circling in the synchrotron. These particles are lost by collision with the walls of the vacuum duct and ancillary apparatus.
After acceleration to a prescribed energy level under the influence of beam instability, the beams of charged particles have the particle distribution in the phase space as shown in FIG. 10. The beam particles circle with betatron oscillation in the horizontal and vertical directions around the design orbit. FIG. 10(A) shows the phase space in the horizontal direction after completion of acceleration, and FIG. 10(B) shows the phase space in the vertical direction after completion of acceleration. The abscissa in FIG. 10 represents the deviation (position P) from the design orbit, and the ordinate in FIG. 10 represents the inclination (angle θ) toward the design orbit. The particle distribution in the beams of charged particles which have undergone acceleration has the core part at the center and the peripheral part called halo. The halo is formed by those beam particles which have increased in the amplitude of oscillation (emittance) in the horizontal and vertical directions under the influence of beam instability. The degree of halo formation has no reproducibility because the degree of beam instability has no reproducibility.
Ejection of beam particles from the synchrotron starts with those beam particles which have a large amplitude of betatron oscillation (emittance) and which have exceeded the stability limit. This suggests that beam particles forming halos (or halo-beam particles) are extracted first out of beam particles circling the synchrotron. This is a probable reason why the extracted beams (or the beams of charged particles extracted from the synchrotron) lacks good reproducibility in emittance immediately after extraction, and hence the reproducibility of the beam size and the stability of the beam orbit are poor in the irradiation apparatus immediately after the start of extraction from the synchrotron. The results of the present inventors' experiments revealed that the beam particles extracted from the synchrotron rapidly decrease in emittance with the lapse of time. A probable reason for this is that the extracted beams rapidly decrease in emittance as the extraction of halo-beam particles proceeds.